Technical Field
The present disclosure relates to a method for demodulating a received signal relating to a sequence of transmitted symbols that have been modulated by continuous phase modulation (CPM) and to an apparatus (receiver) for demodulating a received signal relating to a sequence of transmitted symbols that have been modulated by CPM.
The disclosure is particularly, though not exclusively, applicable to demodulating a received signal relating to a sequence of transmitted symbols that represent one or more messages in an Automatic Identification System (AIS). Embodiments of the disclosure are particularly suited to be applied to an AIS receiver in a spacecraft, such as a satellite.
Description of the Related Art
An AIS provides identification and location information to naval vessels and shore stations with the aim of exchanging data including information on position, identification, course and speed. This allows naval vessels to anticipate and thus avoid collisions with other naval vessels by means of continuous traffic monitoring with several navigation aids. In addition, AIS also offers important naval vessel monitoring services to coastal guards or to search and rescue organizations.
The AIS is based on broadcasting of fixed-length digital messages in a Time Division Multiple Access (TDMA) framework. Individual AIS messages corresponding to sequences of symbols to be transmitted are modulated by means of CPM. Each naval vessel equipped with an AIS apparatus broadcasts information (data) in small slots of 26.67 ms. In each of these slots a message of 256 bits is transmitted at a rate of 9600 b/s using a binary Gaussian Minimum Shift Keying (GMSK) modulation over two Very High Frequency (VHF) carriers. Nearby AIS emitters synchronize with each other in order to avoid packet collisions, i.e., avoid emission of more than one packet in the same time slot by different emitters (time slots are defined globally on the basis of a common temporal reference provided by GPS). As a result, Self-Organized Time Division Multiple Access (SOTDMA) regions are formed. Each SOTDMA region (SOTDMA cell) is designed to cope with path delays not longer than 12 bits, which translates into a maximum range of about 200 nautical miles, but typically the radio frequency coverage is limited to about 40 nautical miles. Within this range all the naval vessels in visibility transmit in accordance with the SOTDMA protocol which ensures that packet collisions between bursts transmitted by different naval vessels are prevented.
Attempts to improve handling of hazardous cargo, security and countering illegal operations have led to the introduction of satellite based AIS. Satellite based AIS enables detecting and tracking naval vessels at distances from coastlines that are larger than can be accomplished by normal terrestrial VHF communications, so that naval vessels may be detected at very long distanced from shores. In particular, a LEO (low earth orbit) constellation of small-size satellites, with an altitude ranging from 600 km to 1000 km, can provide global coverage. Each satellite is provided with an on-board small VHF antenna with a field of view spanning over a few thousands of nautical miles and thus comprising up to several hundreds of SOTDMA cells.
Satellite-based AIS, however, has to face with additional technical challenges that were not considered in the original AIS standard: AIS messages from naval vessels belonging to different SOTDMA cells are not synchronized and therefore can collide with each other, satellite motion with respect to the emitters induces a significant Doppler shift of the carrier frequency, the signal to noise ratio is lower than in terrestrial AIS, and the relative propagation channel delay among the population of naval vessels in visibility at any given time is much higher than for terrestrial AIS.
These problems have been addressed in patent document EP 2 315 366 A1 which relates to a receiver architecture for satellite-based AIS systems. This receiver architecture is composed of three zonal demodulators that process different (but overlapping) frequency bandwidths, as is shown in FIG. 1. The frequency band of each of the AIS channels is sub-divided into three sub-bands, and each of the sub-bands is processed by a corresponding one of the zonal demodulators, thereby exploiting the carrier Doppler diversity for obtaining an estimate of the distance to the respective transmitter and the corresponding path delay. Interference resilient message synchronization is performed by means of Cyclic Redundancy Check (CRC)-aided techniques. Multiple colliding messages are detected by means of digital re-modulation and cancellation of successfully decoded messages.
However, the above solution to the problems faced by satellite-based AIS turns out to be in need of improvement as regards packet error rate (PER) and bit error rate (BER), especially in the presence of heavy traffic leading to heavy interference between AIS messages received at the AIS receiver, and in the presence of AIS messages containing long sequence of zeros. The latter typically occur for latitudes and/or longitudes of the transmitting naval vessel close to zero degrees, i.e., close to the equator and/or the zero median, e.g., in the gulf of Guinea.